The analogy of glass is half empty or half full conveys a very deep-rooted meaning! It indicates that the whole Cosmos has a dual nature. If we have God... we also have Satan (both equal in power... one at plus hundred and the other at minus hundred). We have day... we also have night. We have fire... we also have water. We have plus... we also have minus. All the analogies indicate equal power.
What is it that differentiates the two? One is indicative of the positive power of the Cosmos and the other the negative power of the Cosmos. And in between live human beings and all manifest life. At any given point of time we either invoke positive thoughts from the reservoir of mind plus or negative thoughts from the reservoir of mind minus.
As expressed in the Anekantavada theory of Jainism... both the viewpoints may be correct. We either perceive the glass as half full or half empty. One who dwells in negative thoughts sees the glass half empty. One who dwells in positive thoughts sees the glass as half full. The difference lies only in perception... the object remaining the same!
In the journey of life we find all human beings divided between the two. Some perceive the glass half full and others half empty. Those who perceive the glass half full always live a life of contentment. Those who perceive the glass half empty practice a bickering attitude towards life... they never feel happy come whatever may!
Between success and failure lie our dormant desires. To be successful in life we need to perceive the glass half full... not under pressure but willfully. All optimists view the glass half full... pessimists half empty! If we pray to God... we develop a godly nature in time. Indulging in wanton desires and materialistic riches... one develops satanic nature (a wicked attitude towards others).
To choose and what to choose is the sole prerogative of every human being. More on mind power - http://www.godrealized.org/mind_a_universal_phenomenon.html